Heat transfer systems based on a chemical heat pump are effective to store thermal energy through reaction pairs. They are effective to convert low temperature heat to high temperature heat and are able to achieve coefficients of performance greater than unity when heating and have the additional ability to store thermal energy over long periods with minimum loss and at high energy densities.
By reason of these factors, chemical heat pumps have potential utility in using off-peak energy in power plants, in utilizing solar thermal, ocean thermal, geothermal industrial and nuclear waste thermal energy, in thermal energy storage and in other applications having temporal and spatial energy storage problems.
Chemical heat pumps use chemical reactions of the type: absorbent (dry)+vapor.revreaction.absorbent (wet)+generated heat.
A known system, shown in the U.S. Pat. Nos. to Brunberg et al 4,205,531 of June 30, 1980 and 4,186,794 of Feb. 5, 1980, uses sodium sulfide and water as working substances. In this system, liquid water is disposed in one chamber and the sodium sulfide is disposed in a second chamber which is connected to the first chamber by a pipe which is fitted with a condenser and a valve. The system is brought to an operating pressure less than atmospheric pressure by a vacuum pump. In operation, water is evaporated from the first container, passes through the pipe and take up by the sodium sulfide according to the following equation: EQU Na.sub.2 5.multidot.H.sub.2 O.sub.(s) +4H.sub.2 O.sub.v .revreaction.Na.sub.2 S.multidot.5H.sub.2 O+generated heat
In this reaction, heat is supplied from a low temperature source to vaporize the water from the body of liquid. The water vapor passes through the pipe into the body of sodium sulfide where it reacts to liberate heat at a higher temperature. The reaction is reversible by supplying heat from a high temperature source to the hydrated sodium sulfide to drive off the water as water vapor which passes through the pipe and is condensed and returned to the body of liquid water.
While this system offers many advantages, the working material, sodium sulfide, is classified as a "dangerous substance," comparable to caustic soda and in fact is explosive at high temperatures. Additionally, its heat conductivity is relatively low so that difficulty in transfer of heat to or from the sodium sulfide reduces the effectiveness of the system.
Other reaction pairs such as SrBr.sub.2 .multidot.6H.sub.2 O.revreaction.SrBr.sub.2 .multidot.H.sub.2 O+5H.sub.2 O, FeI.sub.2 .multidot.6NH.sub.3 .revreaction.FeI.sub.2 .multidot.2NH.sub.3 +4NH.sub.3 and FeCl.sub.2 -6NH.sub.3 .revreaction.FeCl.sub.2 .multidot.2NH.sub.3 +4NH.sub.3 have been suggested but present difficulties such as development of corrosive substances, hazards from escaping fumes and costly equipment requirements.
A process and apparatus based on a similar principle but not operating under vacuum conditions effective to remove gases and vapors other than water vapor shows a list of vapor absorptive materials including various metal oxides and chlorides, urea and activated alumina. It is understood that no apparatus according to this patent has been built presumably because the need to eliminate residual gases in the apparatus was not recognized and the presence of these gases interfered with the effective operation.
Various other patents relating to heating or cooling systems include British Pat. No. 1,571,485 which effects cooling by adding liquid water to crystalline urea, German Pat. No. 27 10 287 corresponding to British Pat. No. 1,557,416 shows generation of heat by reacting or dissolving calcium oxide in water and German Pat. No. 22 33 107 which shows cooling hot gases by passing them through fine particles of copper oxalate or aluminum chloride hexahydrate.